GMR sensor having layers treated with nitrogen for increased magnetoresistance

ABSTRACT

A magnetoresistive sensor having a substrate that has been treated with nitrogen (nitrogenated) and having a Ta cap layer with nitrogen added in situ during deposition. The nitrogenated substrate includes an alumina base layer and a thin top layer of crystalline alumina that has had a very small amount of nitrogen deposited on top. The amount of nitrogen deposited on top of the alumina is less than or equal to two monolayer, and is preferably less than on monolayer. The amount of nitrogen deposited on top of the alumina substrate is riot enough to constitute a layer of nitrogen, but affects the structure of the alumina to cause the alumina to have a desired crystalline structure and an extremely smooth surface. The nitrogen in the cap layer can be formed by depositing a Ta cap layer in a sputter deposition chamber having a small amount of nitrogen in an Ar atmosphere.

FIELD OF THE INVENTION

The present invention relates to the manufacture of magnetoresitivesensors and more particularly to the surface treatment of selectedlayers with Nitrogen to improve magnetoresistive performance in thesensor.

BACKGROUND OF THE INVENTION

The heart of a computer is an assembly that is referred to as a magneticdisk drive. The magnetic disk drive includes a rotating magnetic disk,write and read heads that are suspended by a suspension arm adjacent toa surface of the rotating magnetic disk and an actuator that swings thesuspension arm to place the read and write heads over selected circulartracks on the rotating disk. The read and write heads are directlylocated on a slider that has an air bearing surface (ABS). Thesuspension arm biases the slider into contact with the surface of thedisk when the disk is not rotating but, when the disk rotates, air isswirled by the rotating disk. When the slider rides on the air bearing,the write and read heads are employed for writing magnetic impressionsto and reading magnetic impressions from the rotating disk. The read andwrite heads are connected to processing circuitry that operatesaccording to a computer program to implement the writing and readingfunctions.

The write head includes a coil layer embedded in first, second and thirdinsulation layers (insulation stack), the insulation stack beingsandwiched between first and second pole piece layers. A gap is formedbetween the first and second pole piece layers by a gap layer at an airbearing surface (ABS) of the write head and the pole piece layers areconnected at a back gap. Current conducted to the coil layer induces amagnetic flux in the pole pieces which causes a magnetic field to fringeout at a write gap at the ABS for the purpose of writing theaforementioned magnetic impressions in tracks on the moving media, suchas in circular tracks on the aforementioned rotating disk.

In recent read head designs a spin valve sensor, also referred to as agiant magnetoresistive (GMR) sensor, has been employed for sensingmagnetic fields from the rotating magnetic disk. The sensor includes anonmagnetic conductive layer, hereinafter referred to as a spacer layer,sandwiched between first and second ferromagnetic layers, hereinafterreferred to as a pinned layer and a free layer. First and second leadsare connected to the spin valve sensor for conducting a sense currenttherethrough. The magnetization of the pinned layer is pinnedperpendicular to the air bearing surface (ABS) and the magnetic momentof the free layer is located parallel to the ABS, but free to rotate inresponse to external magnetic fields. The magnetization of the pinnedlayer is typically pinned by exchange coupling with an antiferromagneticlayer.

The thickness of the spacer layer is chosen to be less than the meanfree path of conduction electrons through the sensor. With thisarrangement, a portion of the conduction electrons is scattered by theinterfaces of the spacer layer with each of the pinned and free layers.When the magnetizations of the pinned and free layers are parallel withrespect to one another, scattering is minimal and when themagnetizations of the pinned and free layer are antiparallel, scatteringis maximized. Changes in scattering alter the resistance of the spinvalve sensor in proportion to cos Θ, where Θ is the angle between themagnetizations of the pinned and free layers. In a read mode theresistance of the spin valve sensor changes proportionally to themagnitudes of the magnetic fields from the rotating disk. When a sensecurrent is conducted through the spin valve sensor, resistance changescause potential changes that are detected and processed as playbacksignals.

When a spin valve sensor employs a single pinned layer it is referred toas a simple spin valve. When a spin valve employs an antiparallel (AP)pinned layer it is referred to as an AP pinned spin valve. An AP spinvalve includes first and second magnetic layers separated by a thinnon-magnetic coupling layer such as Ru. The thickness of the spacerlayer is chosen so as to antiparallel couple the magnetizations of theferromagnetic layers of the pinned layer. A spin valve is also known asa top or bottom spin valve depending upon whether the pinning layer isat the top (formed after the free layer) or at the bottom (before thefree layer).

The spin valve sensor is located between first and second nonmagneticelectrically insulating read gap layers and the first and second readgap layers are located between ferromagnetic first and second shieldlayers. In a merged magnetic head a single ferromagnetic layer functionsas the second shield layer of the read head and as the first pole piecelayer of the Write head. In a piggyback head the second shield layer andthe first pole piece layer are separate layers.

Magnetization of the pinned layer is usually fixed by exchange couplingone of the ferromagnetic layers (AP1) with a layer of antiferromagneticmaterial such as PtMn. While an antiferromagnetic (AFM) material such asPtMn does not in and of itself have a magnetization, when exchangecoupled with a magnetic material, it can strongly pin the magnetizationof the ferromagnetic layer.

The ever increasing demand for data storage drives researcher tocontinually search for ways to increase data rate and data capacity. Asa result, researcher are continually seeking means for increasing themagnetoresistive performance of magnetic sensors. Such an increase inperformance allow sensors to be constructed smaller resulting indecreased bit length and track width while still producing a useablemagnetic signal from the ever smaller magnetic bits of data.

Therefore, there is a need for a way to increase the magneticperformance such as DR/R of a magnetoresistive sensor. Such an increasewould preferably not require significant additional manufacturingcomplexity, and would preferably result in increased throughput andreduced scrap rate.

SUMMARY OF THE INVENTION

The present invention provides magnetoresistive sensor having improvedmagnetic properties. The sensor is deposited on a substrate thatincludes an alumina base layer and a thin layer of alumina having acrystalline structure formed over the alumina base layer. A small amountof nitrogen deposited onto the thin upper layer of alumina causes it tohave a desired surface structure resulting in improved grain structureand magnetic properties of later deposited layers such as a layer ofantiferromagnetic material (AFM layer). The sensor also includes a caplayer that includes Ta and N. The N in the cap layer can be provided bydepositing the Ta cap in a sputter deposition chamber having a desiredamount of N in an Ar atmosphere.

The may also include seed layers which may be a layer of NiFeCr and alayer of NiFe formed over the alumina substrate. The seed layers, suchas the later deposited NiF may also be nitrogenated by depositing asmall amount of N after depositing the seed layers.

The amount of N deposited onto the thin upper layer of the substrate ispreferably not enough to constitute a layer of N, the amount of N beingno greater than a couple of monolayers. The amount of N is preferablyeven less than a couple of monolayers, and is preferably less than amonolayer, consisting of a scattering of N atoms across the surface ofthe alumina substrate.

The presence of N beneficially affects the surface structure of thesubstrate, and advantageously causes the later deposited sensor layers,such as the AFM layer, to have an improved grain structure an improvedmagnetic properties. The presence of N within the Ta cap layer improvesspecular scattering properties of the cap layer, and acts as a diffusionbarrier, further improving sensor performance.

A sensor having the improved nitrogenated substrate and nitrogenated caplayer has shown significant performance enhancement of about 8 percentas compared with prior art sensors.

These and other features and advantages of the invention will beapparent upon reading of the following detailed description of preferredembodiments taken in conjunction with the Figures in which likereference numerals indicate like elements throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of thisinvention, as well as the preferred mode of use, reference should bemade to the following detailed description read in conjunction with theaccompanying drawings which are not to scale.

FIG. 1 is a schematic illustration of a disk drive system in which theinvention might be embodied;

FIG. 2 is an ABS view of a slider illustrating the location of amagnetic head thereon;

FIG. 3 is an ABS view of a magnetic sensor according to an embodiment ofthe present invention taken from circle 3 of FIG. 2 and rotated 90degrees counterclockwise; and

FIG. 4 is a view taken from circle 4 of FIG. 3, shown enlarged andillustrating a portion of the sensor of FIG. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

The following description is of the best embodiments presentlycontemplated for carrying out this invention. This description is madefor the purpose of illustrating the general principles of this inventionand is not meant to limit the inventive concepts claimed herein.

Referring now to FIG. 1, there is shown a disk drive 100 embodying thisinvention. As shown in FIG. 1, at least one rotatable magnetic disk 112is supported on a spindle 114 and rotated by a disk drive motor 118. Themagnetic recording on each disk is in the form of annular patterns ofconcentric data tracks (not shown) on the magnetic disk 112.

At least one slider 113 is positioned near the magnetic disk 112, eachslider 113 supporting one or more magnetic head assemblies 121. As themagnetic disk rotates, slider 113 moves radially in and out over thedisk surface 122 so that the magnetic head assembly 121 may accessdifferent tracks of the magnetic disk where desired data are written.Each slider 113 is attached to an actuator arm 119 by way of asuspension 115. The suspension 115 provides a slight spring force whichbiases slider 113 against the disk surface 122. Each actuator arm 119 isattached to an actuator means 127. The actuator means 127 as shown inFIG. 1 may be a voice coil motor (VCM). The VCM comprises a coil movablewithin a fixed magnetic field, the direction and speed of the coilmovements being controlled by the motor current signals supplied bycontroller 129.

During operation of the disk storage system, the rotation of themagnetic disk 112 generates an air bearing between the slider 113 andthe disk surface 122 which exerts an upward force or lift on the slider.The air bearing thus counter-balances the slight spring force ofsuspension 115 and supports slider 113 off and slightly above the disksurface by a small, substantially constant spacing during normaloperation.

The various components of the disk storage system are controlled inoperation by control signals generated by control unit 129, such asaccess control signals and internal clock signals. Typically, thecontrol unit 129 comprises logic control circuits, storage means and amicroprocessor. The control unit 129 generates control signals tocontrol various system operations such as drive motor control signals online 123 and head position and seek control signals on line 128. Thecontrol signals on line 128 provide the desired current profiles tooptimally move and position slider 113 to the desired data track on disk112. Write and read signals are communicated to and from write and readheads 121 by way of recording channel 125.

With reference to FIG. 2, the orientation of the magnetic head 121 in aslider 113 can be seen in more detail. FIG. 2 is an ABS view of theslider 113, and as can be seen the magnetic head including an inductivewrite head and a read sensor, is located at a trailing edge of theslider. The above description of a typical magnetic disk storage system,and the accompanying illustration of FIG. 1 are for representationpurposes only. It should be apparent that disk storage systems maycontain a large number of disks and actuators, and each actuator maysupport a number of sliders.

With reference now to FIG. 3, a magnetoresistive sensor 300 according toan embodiment of the invention includes a sensor stack 302. The sensorstack includes a magnetically pinned layer 304, a magnetically freelayer 306 and a non-magnetic, electrically conductive spacer layer 308sandwiched there between. It should be pointed out that although thesensor is being described in terms of a GMR sensor, it could also be atunnel valve (TMR) sensor, in which case the spacer layer 308 would be anon-magnetic, electrically insulating material. A cap layer 309, whichwill be described in greater detail herein below is provide at the topof the sensor and protects the sensor from damage, such as by corrosion,during manufacturing.

The pinned layer 304, may be one of several types of pinned layers, suchas a simple pinned, AP pinned, self pinned or AFM pinned sensor. Forpurposes of simplicity, the sensor will be described herein as an APpinned, AFM pinned sensor having an AP1 layer 310, AP2 layer 312, and anon-magnetic, AP coupling layer, such as Ru 314 sandwiched therebetween. The AP1 and AP2 layers 312, 314 can be constructed of severalmagnetic materials such as, for example NiF or CoFe, and have magneticmoments 316, 318 that are pinned by exchange coupling of the AP1 layer314 with a layer of antiferromagnetic material (AFM layer) 320 such asPtMn.

The AFM layer 320 is preferably built upon a pair of seed layers 322,324. The first seed layer 322, may be for example NiFeCr, and the secondseed layer 324 can be for example NiFe. The seed layers are beneficialin promoting a desired grain structure in the AFM layer 320 formedthereabove. This grain structure substantially carries through to theother magnetic layers formed above the AFM layer 320 as well. The grainstructure of a magnetic material in a magnetoresistive sensor greatlyaffects the magnetic properties of the layers and, therefore, greatlyaffects the performance of the sensor. The present invention addressesperformance improvement through grain structure enhancement, as will bedescribed in more detail below.

The sensor 300 may also include first and second hard bias layers 330,constructed of a hard (high coercivity) magnetic material such asCoPtCr. An in stack bias structure (not shown) may be used in lieu ofthe hard bias layers 330. The hard bias 330 are preferably thick enoughto extend at least to the level of the free layer. The sensor 300 alsoincludes first and second non-magnetic, electrically conductive leads332, formed over the hard bias layer 330, which conduct sense current tothe sensor stack 302. The leads 332 can be for example Cu, Ta, Au orsome other non-magnetic, electrically conductive material.

With continued reference to FIG. 3, the seed layers 322, 324 sit upon afirst gap layer 326, which may be for example Alumina Al₂O₃, and acts asa substrate for the sensor layers deposited thereon. A second gap layer328 is also formed at the top of the sensor and may also be constructedof Al₂O₃. The first and second gap layers 326, 328 electrically insulatethe sensor, preventing current from being shunted around the sensorstack 302.

With reference now to FIG. 4, which shows an expanded view of a portionof the first gap layer 326 and the first and second seed layers 322,324, the first gap layer 326 includes a very thin layer of alumina(Al₂O₃) 402 formed on the substrate/gap layer 326. This thin upper layer402 may have a thickness of 20 to 40 Angstroms or about 30 Angstroms.The underlying gap layer 326 may have an amorphous structure. In otherwords, it is not crystalline. The thin alumina layer 402, by contrasthas a desired crystalline structure, which imparts a desired grainstructure on the later deposited seed layers 322, 324 and other sensorlayers. The crystalline alumina layer 402 has an extremely smoothsurface, which is generated by the inclusion of a very small amount ofnitrogen N, in a novel nitrogen adsorption process that significantlyimproves the magnetic performance of the sensor.

With continued reference to FIG. 4, the very small amount of nitrogen404 can be applied for example by atomic layer deposition, Nitrogenexposure or adsorption of Nitrogen. The amount of nitrogen is preferablytoo small to constitute and actual layer of N. The amount of nitrogenmay be so small as to constitute a scattering of nitrogen atoms acrossthe surface of the crystalline alumina layer 402, or may be enough tofor one or two monolayers of nitrogen on all or a portion of the surfaceof the crystalline alumina layer 402. The thin crystalline alumina layer402 can be deposited for example by atomic layer deposition. Thepresence of the N affects the structure of the surface of the aluminalayer 402 and promotes an improved growth structure in a seed layer 322,324 deposited thereon. This improved seed layer structure results inimproved magnetic properties in the sensor layers deposited over theseed layer, thereby improving sensor performance.

The presence of the nitrogen 404 affects the growth of the seed layers322, 324 and also therefore, advantageously affects the growth of theAFM layer 320. As another embodiment of the invention, one or more ofthe seed layers 322, 324 may be treated with Nitrogen (ie. nitrogenated)as well as the alumina 326. The nitrogenation of the seed layers may insome circumstances provide additional improvement in the microstructureof the AFM 320.

With reference again to FIG. 3, the cap layer 309 preferably includes Tahaving atoms of nitrogen interspersed therein. The nitrogen atoms may beintroduced during deposition of the cap layer 309. For example, the caplayer 309 may be deposited by sputter deposition in a sputter depositionchamber. The sputter deposition chamber may include a Ta target and anatmosphere that includes Ar and N. The resulting cap layer resembles alaminated structure having different material compositions at variouslevels within the cap layer 309. The cap 309 is deposited on a magneticlayer such as the free layer 306, that has been exposed to oxygen. Thisresults in the free layer 306 being, for example, NiFeO. As the Ta and Nare deposited over the free layer the, the portion of the resulting caplayer 309 closest to the free layer pulls oxygen from the free layer 306resulting in a first layer of the cap layer being TaNO. Subsequentlydeposited Ta and N, being further from the free layer does not includethis oxygen and there fore is predominantly TaN, preferably having 30–50atomic percent N or about 40 atomic percent N.

After the cap has been deposited, it is exposed to oxygen, either fromthe atmosphere or from direct oxygenation in a deposition chamber. Thisresults in oxygen replacing the N in the top portion of the cap layerresulting in a top layer of NiO with little or no N. Therefore, theresulting cap 309 is a trilayer laminate structure. The first layer ofthe trilayer cap 309 is predominantly TaNO. The second or middle layerof the cap is FeN with 30–50 atomic percent N or about 40 atomic percentN. The third or top layer of the cap 309 is predominantly TaO, althoughtrace amounts of N may be present. We have found that the abovedescribed cap structure 309 improves specular scattering properties ofthe cap layer 309 and significantly improves performance of the sensor.300.

The presence of nitrogen in the predominantly Ta cap 309 reduces thespecular scattering of electrons passing through the cap 309. As thoseskilled in the art will appreciate, this reduction in spin dependentscattering through the cap layer 309 greatly improves the DR/Rperformance of the sensor. The cap 309 may be constructed so that afirst deposited portion is exposed to O₂ (Ta+O) then a second depositedportion is exposed to N₂ (Ta+N), and then a third portion is againexposed to O₂ (Ta+O). The nitrogen treated cap layer 309 provides animproved diffusion barrier layer, resulting in less O₂ diffusion andtherefore, less of a dead layer in the.

The present invention, having the novel nitrogen doped cap layer 309 andnitrogen adsorption treated alumina layer 402 has shown an 8 percentincrease in DR. This is a very significant performance increase. What'smore, this increase in DR was exhibited without affecting other sensorproperties, so there is no negative trade off to constructing a sensoraccording to the present invention. The nitrogen treatment of thealumina substrate 326 results in improved PtMn crystalline structure. Infact we found that the nitrogenation of the alumina substrate hasresulted in the complete elimination of undesirable 200 phase from thePtMn AFM layer 320. The invention can be quickly and easily incorporatedinto existing manufacturing processes as resulting in negligibledowntime or increased cost. Sensor reliability has also been improved.The present invention also results in improved manufacturing yield duethe improved control of the microstructure of the sensor 300.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Other embodiments falling within the scope of the inventionmay also become apparent to those skilled in the art. Thus, the breadthand scope of the invention should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with the following claims and their equivalents.

1. A magnetoresistive sensor comprising: a substrate, comprising alumina(Al₂O₃), the substrate having been treated with nitrogen; one or moreseed layers; a layer of antiferromagnetic material (AFM layer) depositedover the one or more seed layers; a magnetic pinned layer structureformed over and exchange coupled with the AFM layer; a magnetic freelayer structure; a non-magnetic, spacer layer disposed between the freeand pinned layers a cap layer formed over the free layer, the cap layercomprising N.
 2. A sensor as in claim 1, wherein the cap layer comprisesTa and N.
 3. A sensor as in claim 1, wherein at least a portion of thecap layer comprises Ta and 30 to 50 atomic percent N.
 4. A sensor as inclaim 1, wherein the cap layer includes one or more layers comprising Taand N and one or more layers comprising Ta and O.
 5. A sensor as inclaim 1, wherein the substrate comprises a top layer comprising Al₂O₃and N deposited thereon, the N being in an amount up to two monolayers.6. A sensor as in claim 1, wherein the substrate comprises a top layercomprising Al₂O₃ and N deposited thereon, the N being in an amount lessthan one monolayer.
 7. A sensor as in claim 1, wherein the substratecomprises a thin smooth top layer comprising Al₂O₃ and N in an amountless than one monolayer.
 8. A sensor as in claim 1, wherein thesubstrate comprises a thin smooth top layer comprising Al₂O₃ and N, theN having been deposited after deposition of the Al₂O₃.
 9. A sensor as inclaim 1, wherein the one or more seed layers comprise a layer of NiFeCrand a layer of NiFe formed over the layer of NiFeCr.
 10. A sensor as inclaim 9, wherein at least one of the layers of layer of NiFeCr and thelayer of NiFe includes Ni in an amount up to 2 monolayers.
 11. A sensoras in claim 1, wherein the one or more seed layers comprises a layer ofNiFeCr and a layer of NiFe and N deposited on the NiFe in an amount upto two monolayers.
 12. A sensor as in claim 1, wherein the one or moreseed layers comprises a layer of NiFeCr and a layer of NiFe and Ndeposited on the NiFe in an amount less than a monolayer.
 13. A sensoras in claim 1, wherein the cap layer further comprises: a first layercomprising TaNO; a second layer comprising TaN; and a third layercomprising TaO.
 14. A sensor as in claim 13, wherein the first layer isadjacent to the free layer and the second layer is between the first andsecond layers.
 15. A sensor as in claim 13, wherein the second layer has30 to 50 atomic percent N.
 16. A sensor as in claim 13, wherein thesecond layer has about 40 atomic percent N.
 17. A sensor as in claim 1,wherein the substrate comprises: an Al₂O₃ base layer; a thin crystallinetop layer comprising crystalline Al₂O₃; and atoms of N dispersed acrossa surface of the thin crystalline top layer.
 18. A sensor as in claim17, wherein the base layer is amorphous.
 19. A magnetoresistive sensoras in claim 1, wherein the spacer layer is electrically conductive andthe sensor is a GMR sensor.
 20. A magnetoresistive sensor as in claim 1,wherein the substrate includes base layer comprising Al₂O₃ and a thincrystalline upper layer of Al₂O₃ having a thicknes of 20 to 40Angstroms.
 21. A method of constructing a magnetoresistive sensor,comprising depositing a substrate comprising Al2O3; depositing N in anamount up to two monolayers; depositing a magnetic pinned layerstructure; depositing a spacer layer; depositing a magnetic free layer;depositing a cap layer, in a deposition chamber having an atmospherethat includes N.
 22. A method as in claim 21, wherein the depositionchamber in includes a Ta target and the atmosphere further comprises Ar.23. A method of constructing a magnetoresistive sensor, comprisingdepositing a substrate comprising Al₂O₃; depositing N in an amount lessthan one monolayer; a layer of antiferromagnetic material (AFM layer);depositing a magnetic pinned layer structure; depositing a spacer layer;depositing a magnetic free layer; depositing a cap layer, in adeposition chamber having an atmosphere that includes N.
 24. A method asin claim 23, further comprising after depositing the substrate anddepositing N, depositing one or more seed layers.
 25. A method as inclaim 23 wherein the AFM layer comprises PtMn.
 26. A method as in claim23 further comprising depositing a first seed layer comprising NiFeCr,and depositing a second seed layer comprising NiFe.
 27. A method as inclaim 23 further comprising after depositing the substrate anddepositing N, depositing a first seed layer comprising NiFeCr,depositing a second seed layer comprising NiFe and depositing N in anamount up to two monolayers.